The present invention relates to the art of making castings and, more particularly, to precision casting of fragile and complex shapes.
In general, castings and especially metallic castings are easily removed from the mold if the casting is of a simple shape, i.e., no curved parts or thin walled sections. However, when working with complex shapes such as a bladed rotor, and especially a curved blade airfoil configuratin, removal of the cast shape becomes very difficult and can only be accomplished with very expensive tooling which is not economically feasible in many situations. Presently in the art, wax patterns of the article to be cast are formed by injection molding and thereafter a refractory mass is cast about the patterns. Once the refractory mold is dry the wax is melted leaving a mold with a cavity in the shape of the article to be cast. Once cast, nondestructive mold removal of the low strength cast articles is nearly impossible because of the fragile condition of the cast material.
Accordingly, it is an object of the invention to provide a mold and a process which permits casting of fragile, complex shapes.
It is another object of the invention to provide a process, as described, which is reliable, practicable and economical for production applications.
Briefly, the present imvention discloses a mold and method for making low strength castings through the use of a flexible rubber pattern in the shape of the article to be cast and a two layer wax mold. The rubber pattern is secured to a base plate called a "drag" having a threaded center hole. The mounted pattern is then dipped into a first molten wax to coat the pattern with a first layer of wax. Thereafter, the hardened unit is dipped into a second wax bath of a water soluble wax in order to form a second layer of wax, which together with the first layer, forms a monolithic mold. When the second layer of wax has sufficiently hardened, the pattern is extracted from the two layer wax mold utilizing a puller assembly, which leaves an impression in the mold in the shape of the article to be cast. The mold assembly is then submerged in a water bath to dissolve the water soluble outer layer of the mold assembly. Once dried, the mold is placed on a plaster base and a slip poured into the impression and allowed to solidify. Thereafter, the inner layer of the wax mold is removed from the plaster base by submersion of the mold into an organic solvent which chemically dissolves the first layer of wax, leaving the cast article intact.